Outline

• Layering:
  • OSI
  • Internet
• Ethernet

Layering Principles

• Each layer offers an abstraction of the layers below it.
• Logically, all communication is between peers, protocols at the same layer on different systems.
• Each layer communicates with and only depends on the layer below it and on its peers.

OSI Architecture

• Application: user-level program (Web, e-mail, ftp, telnet)
• Presentation: formatting, encryption, data encoding (ASN.1, XDR)
• Session: establishing the endpoints, identification (TCP, UDP)
• Transport: reliability, end-to-end issues (TCP, UDP)
• Network: basic end-to-end connectivity (IP, ATM AAL)
• Data Link: hop-by-hop connectivity, framing (Ethernet, FDDI, ATM cells)
• Physical: bit transport (fiber, CAT 5, serial)

Internet Architecture

• p. 37 of the textbook
• not strictly layered
• historically came before OSI, or it might have modeled on OSI
• layers:
  • device
  • IP
  • Transport
  • application.
• IP "hourglass" allows universal communication

IETF: Internet Engineering Task Force

• "running code" (multiple implementations) is essential
• theoretical definition sometimes takes a bow to the implementation.
• RFCs (request for comments)
• consensus building through technical co-operation

In-class exercises

• where does the Network File System (NFS) fit in on the OSI model? In the internet model? Does this make sense?
• where does Remote Procedure Call (RPC) fit in on the OSI model? In the internet model? Does this make sense?

Ethernet

• Aloha net + carrier sense + collision detection
• Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
• variety of implementations, including:
  • speeds:
    • 3Mb/s: original
    • 10Mb/s: most popular today
    • 100Mb/s: gaining
    • 1Gb/s: new
  • addresses:
    • 16-bit: original
    • 48-bit: current

Traditional Ethernet Organization

• 48-bit addresses are globally unique
• structure of 48-bit addresses not used in forwarding packets
• broadcast domain: every host sees every packet
• single wire carries signal to all hosts
• sender detecting collision sends a jamming signal, does exponential back-off

Current Ethernet Organization

• hubs: point-to-point wires come in to common bus, where collision detection occurs (logically shared medium)
• to broadcast, hubs must be interconnected with no loops
• switches: point-to-point wires come in to buffered bus, from which packets are sent:
  • only on wire to destination if known (logically partitioned medium)
  • broadcast otherwise (logically shared medium)
• switches might be interconnected with loops (redundant links), so to broadcast, dynamically compute a spanning tree, a loop-free subset that reaches every branch

Ethernet Format

• 64-bit preamble: 010101010101....
• 48-bit destination address (all 1's for broadcast)
• 48-bit source address
• 16-bit type field ( 0x800 for IP, 0x806 for ARP)
• 46 to 1500 bytes of data
• 32-bit CRC
• 8-bit postamble

In-class exercises

1. What is the length of a bit on a 10Mb/s ethernet? assume speed of light of 2/3\ c = 200,000\ km/s
2. Assume that there are 512 bits in a minimally-sized (46-byte payload) ethernet frame. What is the length of this frame?
3. Compute the maximum length of an ethernet, knowing that a frame has to be visible at the same time to both the sender and the farthest receiver.

In-class or at-home exercises

1. what is the size (in bytes) of an ethernet header? (header is source and destination address, and protocol type).
2. Ethernet is often used to carry TCP/IP traffic. The TCP/IP traffic have been carefully designed so all 32-bit fields have 32-bit alignment. If the Ethernet packet carrying TCP/IP traffic is 32-bit aligned, what is the alignment of the TCP/IP headers?
3. Add an additional 16-bit field to the ethernet header. How many bytes is the header now? If the ethernet packet is aligned, what is the alignment of the payload now?
4. Suggest some uses for the new 16-bit field you have added to the ethernet standard.